1. Field of the Invention
The present invention relates to a multilayered circuit substrate for a semiconductor device. Further, the present invention relates to a method of manufacturing the multilayered circuit substrate. Furthermore, the present invention relates to a semiconductor device using such a substrate and a method of producing the same. More particularly, the present invention relates to a multilayered circuit substrate for a semiconductor device having a semiconductor element mounting face on which a plurality of semiconductor elements can be mounted being arranged in the plane direction on one side of the multilayered circuit substrate composed by laminating a plurality of conductor patterns. Further, the present invention relates to a method of producing the multilayered circuit substrate, a semiconductor device, and a method of producing the same.
2. Description of the Related Art
Recently, there has been provided a semiconductor device referred to as a system-in-package (SIP) in which a plurality of semiconductor elements are mounted on one circuit substrate. This semiconductor device will be referred to as “SIP”, hereinafter.
A size of the aforementioned SIP is appropriate to be handled as a chip. An example of SIP is shown in FIGS. 7(a) and 7(b). SIP 100 shown in FIG. 7(a) is composed in such a manner that semiconductor elements 104a, 104b, 104c, the functions of which are different from each other, are mounted in the plane direction on a semiconductor element mounting face formed on one face of a piece of multilayered circuit substrate 102.
As shown in FIG. 7(b), the multilayered circuit substrate 102 forming the above SIP 100 is composed of resin layers 102a, 102b, 102c, 102d which are made of insulating resin, and the conductor patterns 106, 106, . . . and the via holes 108, 108, . . . are formed being laminated on these resin layers 102a, 102b, 102c, 102d. 
On one face of the multilayered circuit substrate 102, there are provided connection pads 110, 110, . . . , from which the connecting faces to be connected with the electrode terminals of the semiconductor elements 104a, 104b, 104c are exposed. On the other face of the multilayered circuit substrate 102, there are provided external connection pads 114, 114, . . . , from which the attaching faces on which the solder balls 112, 112, . . . are attached are exposed.
The connection pads 110, 110, . . . and the external connection pads 114, 114, . . . are electrically connected with each other by the conductor patterns 106, 106, . . . and the via holes 108, 108, . . . which are formed and laminated on the resin layers 102a, 102b, 102c, 102d. 
One face and the other face of the above multilayered circuit substrate are covered with the protective films 116, 118 made of solder resist except for the connection pads 110, 110, . . . and the external connection pads 114, 114, . . . .
In this connection, the potting resin 120 is filled between the semiconductor element mounting face, which is formed on one face of the multilayered circuit substrate 102, and the semiconductor elements 104a, 104b, 104c mounted on the semiconductor element mounting face.
When SIP 100 shown in FIGS. 7(a) and 7(b) are used, the size of SIP can be reduced to be smaller than the size of the device in which a plurality of semiconductor devices, each semiconductor device having a single semiconductor element, are used. Further, it is possible to reduce a conductor distance between the semiconductor elements 104a, 104b, 104c. Therefore, a signal can be sent and received between the semiconductor elements at high transmission speed.
However, SIP 100 shown in FIGS. 7(a) and 7(b) is composed of a multilayered circuit substrate 102 actually made of resin. Therefore, rigidity of the multilayered circuit substrate 102 is not sufficiently high.
Therefore, the present inventors made investigations into SIP 200 shown in FIG. 8(a) and 8(b). As shown in FIG. 8(b), the multilayered circuit substrate 202 composing this SIP 200 is formed by laminating the resin layers 202a, 202b made of insulating resin on which conductor patterns 106, 106 . . . and the via holes 108, 108 . . . are formed. On one face of the multilayered circuit substrate 200, there are provided connection pads 110, 110, . . . , from which the connecting faces to be connected with the electrode terminals of the semiconductor elements 104a, 104b, 104c are exposed.
On the other face of the multilayered circuit substrate 202, the metallic plate 204, which is a plate-shaped member having a rigidity higher than that of the multilayered circuit substrate 202, is bonded by the adhesive layer 206 made of insulating resin.
Since metallic plate 204 is joined to the other face of the multilayered circuit substrate 202, SIP 200 and other electronic parts are electrically connected with each other by a lead frame as shown in FIGS. 8(a) and 8(b). Specifically, an end portion of each inner lead 300, 300, . . . of the lead frame is connected with an exposure face of each external connection pad 208, 208, . . . formed along the outer edge of the multilayered circuit substrate 202 by the wire 302.
In this SIP 200, the highly rigid metallic plate 204 is joined to the other face of the multilayered substrate 202. Therefore, rigidity of this SIP 200 is actually enhanced and higher than that of the multilayered circuit substrate 102 made of resin shown in FIGS. 7(a) and 7(b).
However, since the coefficient of thermal expansion of the multilayered circuit substrate 202 actually made of resin and that of the metallic plate 204 are different from each other, stress is generated between them. Cracks tend to be caused on SIP 200 by the thus generated stress.
Since the multilayered circuit substrate 202 shown in FIGS. 8(a) and 8(b) is successively laminated from the resin layer 202a provided on the metallic plate 204 side. Therefore, on a surface of the resin layer 202b laminated on the resin layer 202a, especially on a surface of the resin layer 202b corresponding to the via hole 108 formed on the resin layer 202a, a concave tends to be formed as shown in FIG. 9. Therefore, the surface of the resin layer 202b tends to become irregular.
When the connection pad 110 to be connected with the electrode terminal of the semiconductor element is formed on the irregular face of the resin layer 202b as shown in FIG. 9, an exposed face of the connection pad 110 is formed into an irregular face, following the irregular face of the resin layer 202b. 
In the case where the exposed face of the connection pad 110 is formed into an irregular face, when a semiconductor element is mounted on a semiconductor element mounting face of the multilayer circuit substrate 202, an electrode terminal of the semiconductor element does not come into contact with the exposed face of the connection pad 110, which causes an imperfect contact and reliability of the finally obtained SIP 200 is deteriorated.
Although the irregular face formed on the resin layer 202b of the multilayer circuit substrate 202 can be flattened by means of polishing, it is necessary to add a polishing process to the conventional manufacturing process of SIP 200, which raises the manufacturing cost of SIP 200. For the above reasons, it is preferable that the semiconductor element mounting face of the multilayered circuit substrate 202 is flattened without adding the polishing process.